This invention relates to disk drives and, more particularly, to a system and method for removing the periodic component of the timing mark position error of a self-servo write (SSW) pattern.
FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a recording slider that includes, for example, an offset head 101, that is positioned over a selected track on a magnetic disk 102 using a servo system for writing data to and/or reading data from the disk 102. Data is recorded in arrays of concentric data information tracks on the surface of the disk 102. While HDD 100 shows only a single magnetic disk 102, HDDs typically have a plurality of stacked, commonly rotated rigid magnetic disks. The servo system of the HDD 100 can include an actuator 105, a voice coil motor (VCM) 104 for coarse positioning a read/write head suspension 106, and a secondary actuator, such as microactuator or micropositioner, for fine positioning the read/write head 101 over a selected track. As used herein a microactuator (or a micropositioner) is a small actuator that is placed between a suspension and a slider and moves the slider relative to the suspension.
Conventional self-servo writing of a hard disk drive is basically performed one track at a time from the inner diameter (ID) to the outer diameter (OD) of a magnetic recording disk, using a spindle motor index and information from previously written nearby tracks. The magneto-resistive (MR) read sensor and the write element of an offset head are not on the same radius, particularly toward the OD of the disk. Accordingly, the offset between the read sensor and the write element changes with respect to tracks on the disk as the skew angle of the armature changes. For example, near the ID of the disk, the offset between the read sensor and the write element typically is a distance of about eight tracks. At the OD of the disk, the offset between the read sensor and the write element typically is a distance of about 40 tracks.
Ideally, a self-servo pattern should be a line nominally following the arc of motion of the head from the ID to the OD of the disk, but because self-servo writing is basically performed by writing the servo pattern one track at a time and because of the offset between the read sensor and the write element, errors propagate through the servo pattern that cause the servo pattern to be warped away from the desired trajectory, which is nominally straight locally. One error pattern that propagates through the self-servo pattern from the ID to the OD of a disk has a period that is related to the offset distance between the read sensor and the write element. To further complicate matters, the periodic error pattern changes as the skew angle of the armature changes. Thus, the warp can become large enough to cause a low-amplitude misalignment or missed windows during a high-speed seek.
U.S. patent application Ser. No. 10/880,850 addresses a similar problem as in the present application, including the warps or shifts in the circumferential direction of the propagated self-servo writing (SSW) timing and/or index marks. This reference discloses determining the periodic component of a timing mark position error of a SSW pattern on a disk based on measurements made of the timing mark position error at selected radial track locations, and removing the determined periodic component of the timing mark position error based on the radial location on the disk. The timing mark position errors of first and second selected radial track locations of a disk are measured. The periodic component of the timing mark position error of the self-servo write pattern is determined based on the timing mark position error of the first selected radial track location and the timing mark position error of the second selected radial track position. The period of the periodic component of the timing mark position error is based on the offset between the read sensor and the write element of the read/write head, and the maximum value of the periodic component of the timing mark position error is based on half of the offset between the read sensor and the write element of the read/write head.
In some prior methods, position error signal (PES) bursts are used to determine the location of the head over disk in the radial direction. The goal is to position the read or write head over the correct data track. All PES bursts are written at the same frequency and phase. In U.S. Pat. No. 5,600,506, two servo bursts are used to get a read or write head over a desired data track. Each data track has a boundary defined by two servo bursts. The data track comprises a gray scale band located in the preamble area of each sector. Each gray scale is divided into two zones. By using a predetermined set of rules regarding the gray scale area address and zone number, any correct radial location between the OD and the ID on a disk can be identified. FIG. 2 shows an example of using PES bursts to determine the location of the head over disk in the radial direction.
Another approach utilizes timing marks based on straight line propagation in the radial direction. In U.S. Pat. No. 6,735,031, previously written timing marks are retrieved to calculate the proper radial location for writing new timing marks. By using the write width, the read to write offset, the read width of the head, and the servo radial step distance in comparison to a timing mark, the radial propagation steps and fractional amount of the read head that lies over part of the timing mark line can be found. Based on a mathematical calculation, future timing marks can be estimated to be written in straight radial lines propagating from the ID to the OD of a disk.
In short, SSW timing propagations can develop warps at a period equal to the head offset, and the warps generally decay due to the effects of read head overlap. This decay can be compromised for narrow read heads and possibly by the normal systematic delay control loop, resulting in persistent warps which can be large enough to cause misalignment (low amplitude) or missed windows during high speed seek. Conventional solutions to the problem are vulnerable to large index noise and are limited in the future by the current levels of index noise. The normally used systematic delay control loop referenced to index is similarly (although less so) vulnerable to the future characteristics of the index noise. Large index noise decreases the ability to accurately determine the self-servowrite warps at a period equal to the head offset.